1. Field of the Invention
The present invention relates to privacy transmission systems in which a communication is rendered unintelligible so that its transmission will be unavailable to unauthorized third persons, the communication being capable of being unscrambled after reception by an authorized person to recover the original intelligence.
2. Description of the Prior Art
Communication systems have been developed which serve to prevent unauthorized persons from intercepting and acquiring the transmitted intelligence communicated therefrom. These systems are based upon a pseudorandom scrambling of the speech signals prior to transmission so as to render the transmission unintelligible and therefore, secure with respect to unauthorized third parties who may intercept the transmission.
Voice communications can be made more secure, or more private, if the speech is mixed with a random process similar to noise; however, the random process cannot be truly random, but must be capable of reproduction, so that the scrambled speech can be descrambled. The mixing may be by multiplication, which results in a signal whose spectrum is determined by the random process, or may be accomplished by shifting segments or blocks of the speech in time, in a random manner. Unfortunately, the former type of system has been found to be disadvantageous in that it requires a rather wide bandwidth; whereas, in the latter type of system, the resulting signal can be made to have the same spectrum as the original speech, a very desirable property, since the scrambled speech will not be confined to radio transmission but can then be sent over telephone lines as well.
A system for providing time division multiplexing of clear speech in order to provide scrambled speech capable of being transmitted with privacy has been disclosed in U.S. Pat. No. 3,824,467, issued July 16, 1974, to Richard Charles French. In the patented system, a plurality of memory segments are randomly addressed by a pseudorandom number generator, and the addressed segments serve to record digital information corresponding to the current time segment of speech while simultaneously recovering stored digital information corresponding to a previous time segment of speech. The scrambled voice segments are descrambled by determining the amount by which each segment has been delayed in the scrambler and then further delaying each segment so that the total delay of each segment of speech, including both the scrambling and unscrambling delays, will be equal.
Another type of system is disclosed in U.S. Pat. No. 3,921,151, issued Nov. 1, 1975, to Gustav Guanella. In this system, an information signal divided into equal time intervals, the individual signal elements being applied in the received sequence to a memory for temporary storage. Each of the stored elements is then read out in a random pattern so as to scramble the arrangement of the elements as compared with their original received arrangement in which they have been stored. The process is effectively reversed at the receiving end to descramble the signal elements.
In spite of the various different privacy systems which have been provided hereinbefore, it has been found that the simple transposition of signal elements provides insufficient security against deciphering, particularly in the case where only a single scrambling of the elements is effected during the reading out of the contents of a memory in which the signal elements have been stored in the order in which they were received. In addition, the conventional pseudorandom number generators heretofore used fail to reduce the periodicity of the generated sequences sufficiently to provide the degree of security required for adequate privacy.
It is therefore a principal object of the present invention to provide a narrow band privacy transmission system having the ability to transmit scrambled voice frequency signals with increased security.
It is another object of the present invention to provide an improved narrow band privacy transmission system of the type wherein blocks or segments of speech are shifted in time in a random manner to effect scrambling of the speech, the scrambling and unscrambling operations being effected automatically in a simplified manner with greater security.
It is a further object of the present invention to provide a system of the type described in which the random nature of the scrambling of speech is materially increased to reduce the periodicity of the generated sequences.